Method for producing cyclopentanone and cyclopentene-1-carboxylic acid and their esters

ABSTRACT

A process for preparing cyclopentanone and cyclopentene-1-carboxylic acid or an ester thereof of the formula I                    
     where R is hydrogen or an aliphatic radical having 1-6 carbon atoms or a cycloaliphatic, araliphatic or aromatic radical having 6-12 carbon atoms comprises heating a compound of the formula II 
     
       
         X—(CH 2 ) 4 —COOR  II 
       
     
     where X is formyl or hydroxymethyl and R is defined as above, and/or a compound which is converted into a compound of the formula II by reaction with water or alcohols ROH under the reaction conditions to from 200 to 450° C. in the gas or liquid phase in the presence of a heterogeneous oxidic catalyst.

The present invention relates to a process for preparing cyclopentanoneand cyclopentene-1-carboxylic acid and an ester thereof by reacting5-formylvaleric acid and an ester thereof and/or 6-hydroxycaproic acidand an ester thereof and/or a compound which is converted into6-hydroxycaproic acid or an ester thereof by reaction of water andalcohols under the reaction conditions, alone or as a mixture withadipic esters, over oxidic catalysts at from 200 to 450° C. in the gasor liquid phase.

EP-A-251 111 discloses the preparation of cyclopentanone by reactingadipic diesters over oxidic catalysts at an elevated temperature in thegas or liquid phase. Furthermore, EP-A-266 687 discloses the use ofzeolitic catalysts or phosphate catalysts for this reaction.

It is an object of the present invention to prepare cyclopentanone fromstarting materials which are even more easily obtainable than adipicdiesters (readily obtainable by esterification of adipic acid), even atthe cost of the coproduction of a further product of value.

This product of value is cyclopentene-1-carboxylic acid or its esters,which have previously been prepared in a rather complicated way byreduction of cyclopentanone-2-carboxylic esters to givecyclopentanol-2-carboxylic esters and subsequent elimination of water(Heterocycles 47 (1996), 423-425.

We have found that this object is achieved according to the invention bya process for preparing cyclopentanone and cyclopentene-1-carboxylicacid or an ester thereof of the formula I

where R is hydrogen or an aliphatic radical having 1-6 carbon atoms or acycloaliphatic, araliphatic or aromatic radical having 6-12 carbonatoms, which comprises heating a compound of the formula II

X—(CH₂)₄—COOR  II

where X is formyl or hydroxymethyl and R is defined as above, and/or acompound which is converted into a compound of the formula II byreaction with water or alcohols ROH under the reaction conditions tofrom 200 to 450° C. in the gas or liquid phase in the presence of aheterogeneous oxidic catalyst.

In a particular embodiment of the process, a mixture of a compound ofthe formula II and an adipic diester of the formula III

ROCO—(CH₂)₄—COOR  III,

where R is defined as above, is reacted, in particular a mixture asobtained by the process according to DE-A 19 607 954.

The reaction according to the invention can be represented, for examplefor the conversion of methyl 5-formylvalerate to cyclopentanone andmethyl cyclopentene-1-carboxylate, by the following reaction equation.

When 6-hydroxycaproic acid or an ester thereof or a compound which isconverted into 6-hydroxycaproic acid or an ester thereof, e.g.ε-caprolactone, an additional simultaneous catalytic dehydrogenation isrequired.

In all cases it was surprising that this reaction proceeded in highyields, selectivities and space time yields.

Starting compounds of formula II are 5-formylvaleric acid and6-hydroxycaproic acid and esters thereof, alone or as a mixture withadipic diesters, in which case the esters may contain aliphatic radicalshaving 1-6 carbon atoms or cycloaliphatic, aromatic radicals oraraliphatic radicals having 5-12, preferably 6-8, carbon atoms. Examplesof radicals R are methyl, ethyl, propyl, isopropyl, butyl, isobutyl,tert-butyl, hexyl, cyclopentyl, cyclohexyl, phenyl or benzyl radicals.

Examples of compounds of the formula II which may be used as startingmaterials include: 5-formylvaleric acid, 6-hydroxycaproic acid, methyl5-formylvalerate, ethyl 5-formylvalerate, isobutyl 5-formylvalerate,cyclohexyl 5-formylvalerate, benzyl 5-formylvalerate, phenyl5-formylvalerate, 6-hydroxycaproic acid, methyl 6-hydroxycaproate,propyl 6-hydroxycaproate, n-butyl 6-hydroxycaproate, cyclopentyl6-hydroxycaproate, phenyl 6-hydroxycaproate, alone or as a mixture withdimethyl adipate, diethyl adipate or di-n-butyl adipate.

It is also possible to use mixtures of compounds of the formula IIfeaturing both formyl and hydroxymethyl groups as starting compounds.

Further possible starting compounds are compounds which are convertedinto compounds of the formula II under the reaction conditions. Forexample, mixtures of caprolactone and water or alcohols may be usedinstead of 6-hydroxycaproic acid or 6-hydroxycaproic esters. If, forexample in the reaction of 6-hydroxycaproic esters according to theinvention, caprolactone is byproduced, it can be separated off andrecycled.

5-Formylvaleric acid to be used as a starting compound may be obtainedby hydroformylation of 3- and 4-pentenoic acid, for example as describedin WO 97/08127. 5-Formylvaleric esters may be obtained byhydroformylation of 3- and 4-pentenoic esters, for example as describedin EP-A 556 681.

6-Hydroxycaproic acid and 6-hydroxycaproic esters are formed, forexample, by hydrolysis or alcoholysis of caprolactone.

In a particular embodiment, mixtures of 6-hydroxycaproic esters andadipic diesters are used as obtained, for example, by the processesdescribed in DE-A 19 607 954, in which case further compounds may bepresent in addition to 6-hydroxycaproic esters and adipic diesters, suchas caprolactone, 6-alkoxycaproic esters, glutaric diesters,5-hydroxyvaleric esters, 2-oxocaproic esters, 1,2-cyclohexanediols,valerolactone, unsaturated adipic diesters, for example dihydromuconicdiesters, 3-hydroxypentanoic esters, 4-oxopentanoic esters and5-oxohexanoic esters. These compounds generally neither adversely affectthe reaction according to the invention nor, surprisingly, give rise toa deterioration in product quality after purification by distillation.

The proportion of adipic diester in the mixture to be reacted istypically up to 95, preferably up to 90, % by weight.

Suitable catalysts are acidic or basic catalysts, but also catalystshaving both acidic and basic properties. When 6-hydroxycaproic acid oran ester thereof is used as starting compound, the catalysts must alsohave dehydrogenating properties.

For the purposes of the present invention, oxidic catalysts are not onlyoxides in the narrow sense but also complex oxygen-containing compoundswhich have intrinsic acidic or basic properties or may be dopedaccordingly. Hence it is also possible to use heteropolyacids, forexample applied to a carrier, zeolites, which are present in the H-formfor acidic activity and which are doped with alkali for basic activity,metal phosphates or compounds such as carbonates or hydroxides which canbe converted into oxides.

Examples of oxidic catalysts are oxides of elements of groups 1-14 ofthe Periodic Table of the Elements or rare earth metal oxides ormixtures thereof. For example, use may be made of alkali metal oxidessuch as sodium oxide, alkaline earth metal oxides, such as magnesiumoxide, calcium oxide, barium oxide, furthermore boron trioxide, aluminumoxide, silicon dioxide, for example in the form of silica gel, fusedsilica, silicates or quartz, furthermore tin dioxide, bismuth oxide,copper oxide, zinc oxide, lanthanum oxide, titanium dioxide, zirconiumdioxide, vanadium oxides, chromium oxides, molybdenum oxides, tungstenoxides, manganese oxides, iron oxides, cerium oxides, neodymium oxidesor mixtures thereof. The catalysts may also be modified by applyingadditives, such as acids (for example phosphoric acids) or bases (forexample sodium hydroxide).

Specific examples are La₂O₃, ZrO_(2′) Cr₂O₃/ZrO₂, CaO/ZnO, MgO/ZnO,K₂O/TiO₂, La₂O₃/Al₂O₃ and ZrO₂—SO₄.

The heteropolyacids to be used according to the invention contain, asessential element, tungsten or preferably molybdenum, which may bepartially replaced by vanadium. If vanadium is used, V:Mo atomic ratiosof 1:6-1:12 are preferred. Examples of central atoms are phosphorus,silicon, arsenic, germanium, boron, titanium, cerium, thorium,manganese, nickel, tellurium, iodine, cobalt, chromium, iron, gallium,vanadium, platinum, beryllium and zinc. Phosphorus and silicon arepreferred. A preferred ratio of molybdenum or tungsten atoms to therespective central atom is 2.5:1-12:1, preferably 11:1-12:1.

Specific examples of molybdenum-containing heteropolyacids are thefollowing compounds:

dodecamolybdophosphoric acid (H₃PMO₁₂O₄₀ *n H₂O),

dodecamolybdosilicic acid (H₄SiMo₁₂O₄₀ *n H₂O),

dodecamolybdoceric(IV) acid (H₈CeMo₁₂O₄₂ *n H₂O),

dodecamolybdoarsenic(V) acid (H₃AsMo₁₂O₄₂ *n H₂O),

hexamolybdochromic(III) acid (H₃CrMo₆O₂₄H₆ *n H₂O),

hexamolybdonickelic(II) acid (H₄NiMo₆O₂₄H₆ *5 H₂O),

hexamolybdoiodic acid (H₅JMo₆O₂₄ *n H₂O),

octadecamolybdodiphosphoric acid (H₆P₂Mo₁₈O₆₂ *11 H₂O),

octadecamolybdodiarsenic(V) acid (H₆As₂Mo₁₈O₆₂ *25 H₂O),

nonamolybdomanganic(IV) acid (H₆MnMo₉O₃₂ *n H₂O),

undecamolybdovanadophosphoric acid (H₄PMo₁₁VO₄₀ *n H₂O),

decamolybdodivanadophosphoric acid (H₅PMo₁₀V₂O₄₀ *n H₂O),

hexamolybdohexatungstophosphoric acid (H₃PMo₆W₆O₄₀ *n H₂O).

It is of course also possible to use mixtures of heteropolyacids.Preference is given to using dodecamolybdophosphoric acid anddodecamolybdosilicic acid.

As well as free heteropolyacids, it is also possible, however, to employtheir salts, in particular their alkali metal and alkaline earth metalsalts, as catalysts. Preference is given to cesium salts. As with thefree acids, corresponding mixtures of their salts may be used.

The heteropolyacids and their salts are known compounds and can beprepared by known methods, for example by the methods described inBrauer (Editor): Handbuch der Praparativen Anorganischen Chemie, VolumeIII, Enke, Stuttgart, 1981 or by the methods described in Top. Curr.Chem. 76 (1978), 1. Particular preference is given to preparationmethods in which no organic solvent is used and which are carried out inaqueous solution instead.

The heteropolyacids prepared in this manner are generally in hydratedform and are free from coordinatively bound water present therein priorto use. This dehydration can advantageously be carried out thermally,for example by the process described in Makromol. Chem. 190 (1989) 929.Depending on the heteropolyacid used, another possible method ofdehydration is to dissolve the heteropolyacid in an organic solvent, forexample in a dialkyl ether or alcohol, displace the water with theorganic solvent from its coordinate bond with the heteropolyacid andremove the water azeotropically with the solvent.

Typically, anhydrous heteropolyacids prepared by these methods aresubsequently calcined at from 250 to 500° C., preferably from 280 to400° C. Depending on the temperature and pressure selected, theheteropolyacids are typically calcined for from 1 hour to 24 hours. Thecatalysts obtained in this manner can be used directly in the process ofthe invention.

The heteropolyacid catalysts are preferably applied to a support. Tothis end, the heteropolyacid is applied to a support material such asactive carbon, silicon dioxide, titanium dioxide or zirconium dioxide bymethods known per se, for example by impregnating the relevant supportmaterial with a solution of the heteropolyacid in a solvent, preferablywater, and subsequently drying under reduced pressure at from 100 to250° C., preferably from 130 to 250° C., until water can no longer bedetected in the catalyst. Anhydrous heteropolyacids prepared by thesemethods are subsequently calcined at temperatures of from 250 to 500°C., preferably from 280 to 400° C.

Suitable zeolites include any zeolites having basic or acidic centers.

In the case of zeolites having basic properties, zeolites containingalkali metals or alkaline earth metals are used, for example; in thecase of zeolites having acidic properties, zeolites in the acidic H-formare used, in which the alkali metal ions are replaced by hydrogen ions.

Preference is given to 12-ring zeolites of the structure type BETA, Y,EMT and Mordenite, and 10-ring zeolites of the Pentasil type. As well asthe elements aluminum and silicon, zeolites can also contain boron,gallium, iron or titanium in their framework. Furthermore, they can alsobe partially ion-exchanged with elements of the groups 3 and 8 to 13 andthe lanthanide elements.

Zeolites to be used as catalyst include zeolites of the structure typeMFT, MEL, BOG, BEA, EMT; MOR, FAU, MTW, LTL, NES, CON or MCM-22according to the structure classification given in W. M. Meier, D. H.Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier,4^(th) ed., 1996.

Particular examples are the zeolites ZBM-20, Fe—H-ZSM5, Sn-beta zeolite,beta zeolite, Zr-beta zeolite, H-beta zeolite, H-mordenite, USY, Ce—Vzeolite, H—Y zeolite, Ti/B-beta zeolite, B-beta zeolite or ZB-10.

To obtain very high selectivity, high conversions and long times onstream, it is advantageous to modify the zeolites. A suitable method ofmodifying the catalysts comprises, for example, doping the shaped orunshaped zeolites with metal salts by ion exchange or impregnation. Themetals used are alkali metals such as Li, Cs, K, alkaline earth metalssuch as Mg, Ca, Sr. Metals of main groups III, IV and V, such as Al, Ga,Ge, Sn, Pb or Bi, transition metals of subgroups IV-VIII such as Ti, Zr,V, Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Sr, Ni, Pd, Pt, transitionmetals of subgroups I and II such as Cu, Ag or Zn, and rare earth metalssuch as La, Ce, Pr, Nd, Er, Yb and U.

Doping is advantageously carried out by introducing the shaped zeoliteinto a riser pipe and passing an aqueous or ammoniacal solution of ahalide or nitrate of the abovementioned metals over it at from 20 to100° C. Such an ion exchange can take place with the hydrogen, ammoniumor alkali metal form of the zeolite. Another way of applying metal tothe zeolite comprises impregnating the zeolitic material, for examplewith a halide, nitrate or oxide of one of the abovementioned metals inaqueous, alcoholic or ammoniacal solution. Both ion exchange andimpregnation are followed at least by a drying operation oralternatively by another calcination.

A possible embodiment comprises for example dissolving Cu(NO₃)₂×3 H₂O orNi(No₃)₂×6 H₂O or Ce(NO₃)₃×6 H La(NO₃)₂×6 H₂O or Cs₂CO₃ in water andimpregnating the shaped or unshaped zeolite with this solution for acertain period of time, for example 30 minutes. Water is removed fromany supernatant solution in a rotary evaporator. The impregnated zeoliteis then dried at about 150° C. and calcined at about 550° C. Thisimpregnating step can be carried out several times in succession untilthe desired metal content is obtained.

It is also possible to prepare an aqueous Ni(NO₃)₂ solution or anammoniacal Pd(NO₃)₂ solution and to suspend the pure pulverulent zeolitetherein at from 40 to 100° C. by stirring for about 24 hours. Afterfiltration, drying at about 150° C. and calcination at about 500° C.,the zeolitic material thus obtained can be further processed with orwithout a binder into extrudates, pellets or fluidizable material.

An ion exchange of the zeolite present in the H-form or ammonium form oralkali metal form can be carried out by introducing the zeolite in theform of extrudates or pellets into a column and passing, for example, anaqueous Ni(NO₃)₂ solution or ammoniacal Pd(NO₃)₂ solution over it in arecycle loop at a slightly elevated temperature of from 30 to 80° C. forfrom 15 to 20 hours. This is followed by washing out with water, dryingat about 150° C. and calcination at about 550° C. With some metal-dopedzeolites, for example Pd-, Cu- or Ni-doped zeolites, an aftertreatmentwith hydrogen is advantageous.

A further method of modifying the zeolite comprises treating the shapedor unshaped zeolitic material with an acid such as hydrochloric acid,hydrofluoric acid or phosphoric acid and/or steam, advantageously, forexample, by treating the zeolite in pulverulent form with 1N phosphoricacid at 80° C. for 1 hour. The treatment is followed by washing withwater, drying at 110° C./16 h and calcining at 500° C./20 h.Alternatively, before or after being shaped with a binder, zeolites aretreated for example at from 60 to 80° C. with from 3 to 25% strength byweight, in particular from 12 to 20% strength by weight, aqueoushydrochloric acid for from 1 to 3 hours. Afterwards, the zeolite thustreated is washed with water, dried and calcined at from 400 to 500° C.

Further catalysts for preparing cyclopentanone are phosphates, inparticular aluminum phosphates, silicon aluminum phosphates, ironaluminum phosphates, cerium phosphate, zirconium phosphates, boronphosphate, iron phosphate, calcium phosphate or mixtures thereof.

Suitable aluminum phosphate catalysts for the process according to theinvention are in particular aluminum phosphates synthesized underhydrothermal conditions. Examples of suitable aluminum phosphate includeAPO-5, APO-9, APO-11, APO-12, APO-14, APO-21, APO-25, APO-31 and APO-33.

AlPO₄-5(APO-5) is synthesized, for example, by homogeneously mixingorthophosphoric acid with pseudoboehmite (Catapal SB®) in water, addingtetrapropylammonium hydroxide to this mixture, and then reacting in anautoclave under autogenous pressure at about 150° C. for from 20 to 60h. The AlPO₄ is filtered off, dried at from 100 to 160° C. and calcinedat from 450 to 550° C. AlPO₄-9 (APO-9) is likewise synthesized fromorthophosphoric acid and pseudoboehmite, but in aqueous DABCO solution(1,4-diazabicyclo-(2,2,2)-octane) at about 200° C. under autogenouspressure in the course of from 200 to 400 h. If ethylenediamine is usedin place of DABCO solution, APO-12 is obtained.

AlPO₄-21 (APO-21) is synthesized from orthophosphoric acid and pseudoboehmite in aqueous pyrrolidine solution at from 150 to 200° C. underautogenous pressure in the course of from 50 to 200 h.

The process according to the invention can also be carried out withknown silicon aluminum phosphates such as SAPO-5, SAPO-11, SAPO-31 andSAPO-34. These compounds are prepared by crystallization from aqueousmixture at from 100 to 250° C. and under autogenous pressure in thecourse of from 2 hours to 2 weeks, the reaction mixture, comprising asilicon, an aluminum and a phosphorus component, being converted in anaqueous solution comprising amine.

SAPO-5 is obtained, for example, by mixing a suspension of SiO₂ in anaqueous tetrapropylammonium hydroxide solution with an aqueoussuspension of pseudoboehmite and orthophosphoric acid and then reactingat from 150 to 200° C. under autogenous pressure in a stirred autoclavefor from 20 to 200 h. The powder is filtered off, dried at from 110 to160° C. and calcined at from 450 to 550° C. Suitable silicon aluminumphosphates also include ZYT-5, ZYT-6, ZYT-7, ZYT-9, ZYT-11 and ZYT-12. Aprecipitated aluminum phosphate can also be used in the process as aphosphate catalyst.

For example, such an aluminum phosphate is prepared by dissolving 92 gof diammonium hydrogen phosphate in 700 ml of water. 260 g of Al(NO₃)₃×9H₂O in 700 ml of water are added dropwise to this solution in the courseof 2 h, during which pH 8 is maintained by adding 25% strength NH₃solution at the same time. The resulting precipitate is subsequentlystirred for 12 hours and then filtered off with suction and washed. Itis dried at 60° C./16 h.

A boron phosphate catalyst for use in the process according to theinvention can be prepared, for example, by mixing and kneadingconcentrated boric acid and phosphoric acid and subsequently drying andcalcining in an inert gas, air or steam atmosphere at from 250 to 650°C., preferably at from 300 to 500° C.

CePO₄ is obtained by precipitating from 52 g of Ce(NO₃)₃×6 H₂O and 56 gof NaH₂PO₄×2 H₂O. The material is filtered off and shaped intoextrudates, which are dried at 120° C. and calcined at 450° C. Suitablephosphates for the process according to the invention also includeSrHPO₄, FePO₄ and Zr₃(Po₄)₄.

The catalysts described here can be used, for example, in the form offrom 2 to 4 mm extrudates or as tablets having a diameter of, forexample, from 3 to 5 mm, or as granules having particle sizes of, forexample, from 0.1 to 0.5 mm, or in a fluidizable form.

When hydroxycaproic acid is used, additional metals such as copper orsilver on oxidic carriers such as metal oxides are typically used toprovide the catalysts with dehydrogenation activity.

It has been found that the use of basic catalysts leads to increasedformation of cyclopentanone, whereas the use of catalysts having acidicproperties leads to increased formation of cyclopentene-1-carboxylicesters I.

The reaction according to the invention may be carried out withoutwater. The addition of water leads to an increased selectivity and timeon stream. The molar ratio of starting compound II to water isadvantageously 1:0-1:20, in particular 1:0.1-1:5.

The reaction can be carried out in the gas or liquid phase with orwithout diluents. Examples of suitable diluents are solvents which arecompletely or substantially inert under the reaction conditions, forexample ether such as dioxane or tetrahydrofuran and alcohols such asmethanol and ethanol. A gas stage procedure is preferred, provided thateasily volatilizible starting materials are used.

The reaction can be carried out batchwise or continuously as a fixed bedreaction with a fixed bed catalyst, for example in an upflow or downflowprocess in the liquid or gas phase, or as a fluidized bed reaction withthe catalyst in the fluidized state in the gas phase, or with a catalystsuspended in the liquid phase.

The reaction is carried out at from 200 to 450° C., preferably from 250to 390° C., in particular from 300 to 380° C. The reaction is generallycarried out under atmospheric pressure. However, it is also possible touse slightly reduced or slightly elevated pressure, for example up to 20bar. The space velocity is generally in the range from 0.01 to 40,preferably from 0.1 to 20, g of compound of the formula II per gram ofcatalyst per hour.

The liquid-phase reaction is carried out, for example, by heating amixture of the starting compound II with or without water to the desiredreaction temperature in the presence of a suspended fixed-bed catalyst.After the required reaction time, the reaction mixture is cooled downand the catalyst removed, for example by filtration. The reactionmixture is then subjected to a fractional distillation to recover theproducts of value and the unconverted ester. The reaction productsformed in the course of the reaction can also be continuously removedfrom the reaction mixture by distillation.

In a preferred embodiment of the process according to the invention inthe gas phase, a mixture of starting compound II with or without wateris initially vaporized and then passed, with or without hydrogen or aninert gas, such as nitrogen, carbon dioxide or argon, in gaseous forminto a fluidized catalyst bed at the desired reaction temperature.

In another preferred embodiment of the process according to theinvention in the gas phase, for example, a mixture of the startingcompound II with or without water is initially vaporized and thenpassed, with or without an inert gas, such as nitrogen, carbon dioxideor argon, in gaseous form over a fixed catalyst bed in an upflow ordownflow process at the desired reaction temperature.

The reaction effluent is condensed by means of suitable cooling devicesand then worked up by fractional distillation. Unconverted startingcompounds may be recycled.

Cyclopentanone obtained by the process of the present invention is auseful intermediate. For instance, reductive amination givescyclopentylamine which is of interest for the synthesis of cropprotection agents and pharmaceuticals.

Cyclopentene-1-carboxylic esters are useful building blocks for thesynthesis of intermediates.

EXAMPLE

The percentages indicated to characterize the catalysts are by weight.

a) Catalyst Preparation:

ZrO₂: obtained from Norton (SN 9516321); directly used as such. La₂O₃(3% of La)/ZrO₂ ZrO₂ (Norton, SN 9516321) was impreg- nated withLa(NO₃)₃ solution, dried at 120° C. for 4 hours and calcined at 400° C.for 6 hours. La₂O₃ (10% of La)/(α-Al₂O_(3:) α-Al₂O₃ (Norton) wasimpregnated with La (NO₃)₃ solution, dried at 120° C. for 4 hours andcalcined at 400° C. for 6 hours. 95% of ZnO/5% of MgO obtained from BASF(H5-10) 56% of ZnO/44% of CaO obtained from BASF (H5-11) Al₂O₃ obtainedfrom BASF (D 10-10)

b) Experimental Procedure for the Examples of Table 1:

100ml of a catalyst were covered with 30 ml of quartz rings asvaporizing section in a gas phase reactor. 10 g of starting material,corresponding amounts of water (Table 1), methanol or 50 standard litersof nitrogen, respectively (Examples 18-22), or hydrogen (Examples 1-17)were passed over a catalyst in a downflow procedure at the indicatedtemperatures. The reaction effluents were condensed in a receiver usingdry ice/acetone. The compositions of the reaction effluent and thus theconversions and selectivities were determined by gas chromatography(Table 1).

TABLE 1 Starting H₂O Temp. Conversion Selectivity [%] Example materialCat. [moles]^(e) [° C.] [%] CPO^(a) MECPC^(b)  1 ME − 5 - FV^(c) La₂O₃(3% of La) ZrO₂ 0 300 52 33.1 38.5  2 0 350 99 38.5 14  3 ZrO₂ 0 270 4011.5 68.3  4 0 300 48.5 14.2 68.3  5 0 330 68.9 19.5 60.5  6 95% ofZnO/5% of MgO 0 270 28.1 51.9 2.0  7 0 300 16.6 59.3 1.8  8 0 330 15.063.8 6.7  9 56% of ZnO/44% of CaO 0 270 16.2 59.7 4.6 10 0 300 30.3 38.92.1 11 0 330 47.4 43.8 2.0 12 Al₂O₃ 0 270 48.6 23.0 17.6 13 0 300 47.324.7 48.9 14 0 330 61.7 34.2 24.0 15 La₂O₃ (10% of La)/ 0 300 35.7 61.03.9 16 α-Al₂O₃ 0 330 42.0 69.0 4.7 17 0 360 62.0 65.7 3.1 18 ME − 6 -HC^(d) 56% of ZnO/44% of CaO 6 350 90.1 40.7 4.3 19 6 380 91.1 72.6 2.420 0 380 97.1 41.8 3.9 21 Caprolactone 56% of ZnO/44% of CaO 3 MeOH 38065.6 47.9 15.5 22 Caprolactone 95% ZnO/5% MgO 3 MeOH 380 69.8 32.7 25.8^(a)CPO = Cyclopentanone; ^(b)MECPC = Cyclopentene-1-carboxylic acid andits methyl ester; ^(c)ME − 5-FV = Methyl 5-formylvalerate; ^(d)ME − 6-HC= Methyl 6-hydroxycaproate; ^(e)based on moles of ester or caprolactoneused

Experimental Procedure for the Examples in Table 2:

The starting materials indicated in Table 2 are obtained from stage 12of the process described in DE-A 19 607 954. 100 ml of a catalystcomposed of 56% by weight of ZnO and 44% by weight of CaO (BASF: H 5-11)were covered with 30 ml of quartz rings as vaporizing section in a gasphase reactor. 10 g of the starting materials indicated in Table 2 and20 standard liters of nitrogen were passed over the catalyst in adownflow procedure at 380-400° C. The reaction effluents were condensedin a receiver using dry ice/acetone. The reaction effluent was collectedover a period of 7 hours. The stated cyclopentanone selectivity is basedon dimethyl adipate, methyl 6-hydroxycaproate and, if used, caprolactoneconverted.

TABLE 2 H₂O added (moles Cyclopentanone Starting per mole of TemperatureConversion selectivity Example material Catalyst starting material) [°C.] [%] [%] 23 ME − 6-HC 56% of ZnO, 3 380 98.1 67.7 24 1:1¹⁾ 44% of CaO93.5 90.0 25 2:1¹⁾ 92.5 67.3 26 4:1¹⁾ 94.8 61.5 27 Ester mixture²⁾ 79.468.4 28 0 29.2 76.4 29 6 400 66.9 82.2 DMA = Dimethyl adipate; ME − 6-HC= Methyl 6-hydroxycaproate ¹⁾Molar ratio DMA:ME − 6-HC ²⁾Ester mixture:% by weight Dimethyl adipate 59.2 Methyl 6-hydroxycaproate 14.6Caprolactone 1.4 Methyl 6-methoxycaproate 0.3 Dimethyl glutarate 6.7Methyl 2-oxocaproate 2.2 Methyl 5-hydroxyvalerate 2.0 Dimethyldihydromuconate 2.4 1,2-cyclohexanediol 0.5 δ-valerolactone 2.4

We claim:
 1. A process for preparing cyclopentanone andcyclopentene-1-carboxylic acid or an ester thereof of the formula I

where R is hydrogen or an aliphatic radical having 1-6 carbon atoms or acycloaliphatic, araliphatic or aromatic radical having 6-12 carbonatoms, which comprises heating a compound of the formula IIX—(CH₂)₄—COOR  II where X is formyl or hydroxymethyl and R is defined asabove, and/or a compound which is converted into a compound of theformula II by reaction with water or alcohols ROH under the reactionconditions to from 200 to 450° C. in the gas or liquid phase in thepresence of a heterogeneous oxidic catalyst.
 2. A process as claimed inclaim 1, wherein a compound of the formula II is reacted in a mixturewith an adipic diester of the formula III ROCO—(CH₂)₄—COOR  III, where Ris defined as above.
 3. A process as claimed in claim 1, wherein thecompound which is converted into a compound of the formula II isε-caprolactone.
 4. A process as claimed in claim 1, wherein the startingmaterial used is a mixture of 6-hydroxycaproic ester and adipic diester.5. A process as claimed in claim 1, wherein water is added to thereaction mixture.
 6. A process as claimed in claim 1, wherein thecatalyst used when using hydroxycaproic acid or a hydroxycaproic esterhas additional dehydrogenation activity.
 7. A process as claimed inclaim 1, wherein a basic oxidic catalyst is used to give predominantlycyclopentanone.
 8. A process as claimed in claim 1, wherein an acidicoxidic catalyst is used to give predominantly cyclopentene carboxylicacid or an ester thereof.
 9. A process as claimed in claim 1, whereinthe catalyst used is a zeolite.
 10. A process as claimed in claim 1,wherein the catalyst used is a heteropolyacid.
 11. A process as claimedin claim 1, wherein the catalyst used is a phosphate.
 12. A process asclaimed in claim 1, wherein the catalyst used is a metal oxide of groups1-14 of the Periodic Table of the Elements and/or a lanthanide oxide.